Fetoscopic visualization may be required for the prenatal diagnosis of external fetal malformations which cannot be clearly defined by ultrasound. The synchronous use of real-time ultrasound guidance during the fetoscopic procedure has permitted successful visualization to be achieved in pregnancies of greater than 18 weeks' gestation. The chances of successful visualization and the influence of advancing gestational age have been critically evaluated. Between 18 and 23 weeks', when these techniques were used, advancing gestational age did not appear to exert a significant effect on the chances of successful visualization. Analysis of placental position indicated that an anterior placenta did not significantly affect bloodstaining of the fluid or preclude access to placental vessels for blood sampling.